Semiconductor devices are fabricated using a succession of spatially related patterns that are transferred to a substrate using lithography. In order to ensure that the devices perform as expected, successive patterns need to be aligned to within a particular alignment tolerance.
In order to align the wafer with the mask or reticle, some lithography systems use through the lens (TTL) alignment techniques. FIG. 1 shows a schematic of a lithography system 100 using TTL alignment. A reticle 110 includes an alignment grating 120 with a pitch P. A wafer 150 to be aligned with reticle 110 includes a previously fabricated alignment pattern 140, also with pitch P. Alignment pattern 140 is generally positioned below a top surface of wafer 150. Note that system 100 may include additional optical elements between reticle 110 and wafer 150, such as one or more lenses.
Grating 120 is illuminated using a light source 130. Light transmitted through grating 120 is incident on wafer 150. The incident light at wafer 150 has the same periodicity as that of grating 120. That is, the distance between intensity maxima of the incident light is equal to the pitch P of grating 120.
At least some of the light is reflected by wafer 150. For example, portions of the incident light which are incident on the features of alignment pattern 140 are strongly reflected, while portions of the incident light which are incident on areas between the features of alignment pattern 140 are less strongly reflected (i.e., a higher proportion of the incident light is absorbed in the areas between the features of alignment pattern 140). Light reflected from wafer 150 is then detected; for example, a portion of the light reflected from the surface of wafer 150 is directed to a detector 170 using a partial mirror 160.
To determine the optimum alignment, the position of wafer 150 is changed using a wafer stage 155. When grating 120 and pattern 140 are aligned, the signal detected with detector 170 is maximum. The signal received by detector 170 may be indicative of a reflected light intensity and/or a interference pattern of the reflected light.
To detect an interference pattern, mirror 160 may be a beam splitter, as shown in FIG. 1. Light 132 is incident on mirror 160. A reference portion 133 is reflected toward detector 170. A transmitted portion 134 continues toward wafer 150 and is at least partially reflected by alignment pattern 140. Mirror 160 reflects a signal portion 135 of the light reflected by alignment pattern 140 to detector 170. At detector 170, reference portion 133 and signal portion 135 produce an interference pattern, which varies as the position of wafer 150 is varied with respect to reticle 110. When grating 120 and alignment pattern 140 are aligned, the interference pattern exhibits constructive interference.
The resolution of the alignment system depends on a number of factors, such as the numerical aperture of the optical system, the wavelength of light used, and the pitch of the grating and alignment features. However, constraints on these parameters may affect the obtainable resolution.
For example, constraints on the wavelength of light used for alignment may place limits on the resolution of the system. Long wavelengths are typically used for two reasons: first, so that the light used to align the lithography system does not expose the photoresist, and second, so that the light is not overly absorbed in the material forming the alignment features on the wafer. Typical alignment wavelengths include 533 nm and 632 nm.
Choosing a particular alignment wavelength places constraints on the pitch of the alignment grating that may be used. For an optical system with a numerical aperture NA, a particular alignment wavelength λ corresponds to a minimum alignment grating pitch of P=λ/NA. Smaller pitches are below the resolution limit, and thus the pattern fidelity is degraded.
Like reference symbols in the various drawings indicate like elements.